1. Field of the Invention
The present invention relates to a photovoltaic device utilizing a hetero semiconductor junction and a manufacturing method thereof.
2. Description of Related Art
A photovoltaic device is generally categorized into single crystalline-, polycrystalline-, and amorphous-based type depending on a type of a semiconductor of a part for converting absorbed light mainly into electric current. By the way, a hybrid type photovoltaic device taking both advantages of an amorphous semiconductor thin film and a crystalline semiconductor is studied (JP, H04-130671, A). In this photovoltaic device, an intrinsic amorphous silicon thin film without dopant of a conductive impurity or a substantially intrinsic amorphous silicon thin film doped with a small amount of an element in the 3A group of the periodic table such as boron is interposed on a junction interface in forming a semiconductor junction by combining a crystalline silicon semiconductor and an amorphous silicon semiconductor having different conductivities from each other so as to improve interface characteristics and photovoltaic conversion characteristics.
A p-n junction of this structure can be formed at a temperature lower than 200 degrees centigrade. Therefore, good interface characteristics can be achieved even with an impure substrate, which is subject to an adverse effect of impurity or deficiency of oxygen induction under a high temperature process.
A substantially intrinsic amorphous silicon thin film without dopant of a conductive impurity or a substantially intrinsic amorphous silicon thin film, and an amorphous silicon thin film made to be one conductive type or another conductive type by doping a one conductive type or another conductive type impurity are formed on a rear surface of the above-mentioned photovoltaic device to improve solar cell characteristics by BSF (Back Surface Field) effect.
FIG. 8 is a perspective view illustrating a photovoltaic device comprising a single crystalline silicon substrate with a textured surface, wherein a substantially intrinsic (i-type) amorphous semiconductor layer without dopant of a conductive impurity is interposed on a junction interface between the crystalline semiconductor and an amorphous semiconductor. As shown in the Fig., a front surface of an n-type single crystalline silicon (Si) substrate 101 is textured by alkaline etching or the like. An i-type amorphous silicon (a-Si) layer 102, a p-type amorphous silicon (a-Si) layer 103 and a transparent electrode 104 consisting of a transparent conductive film such as an ITO (Indium Tin Oxide) film are laminated in this order on the textured surface on a light receiving surface side of the single crystalline silicon substrate 101. Additionally, comb-shaped collector electrodes 105 consisting of silver (Ag), for example, are formed on the transparent electrode 104.
An i-type amorphous silicon (a-Si) layer 106, an n-type amorphous silicon (a-Si) layer 107 and a transparent electrode 108 consisting of a transparent conductive film such as an ITO (Indium Tin Oxide) film are laminated in this order on a rear surface of the single crystalline silicon substrate 101 to achieve a BSF effect. Additionally, comb-shaped collector electrodes 109 consisting of silver (Ag), for example are formed on the transparent electrode 108.
With the above structure, light reflection is suppressed on the textured surface so that light can be taken into the device effectively.
Meanwhile, the above-mentioned photovoltaic device can be formed under a low temperature process. Because of this low temperature process, it is difficult to remove water or organic matters on the surface of the substrate completely. Therefore, impurities such as oxygen, nitrogen or carbon exist on the surface of the substrate. The one most largely included among these impurities is oxygen having a density 1×1020 cm−3. Degraded interface characteristics caused by these impurities may adversely affect the p-n junction characteristics or BSF effect.
On the other hand, a method to achieve a substantially intrinsic amorphous silicon film by doping a small amount of boron is reported (Applied Physics Letters vol.68, 1996 p1201˜p1203). According to this report, it is possible to compensate adverse effect of oxygen by introducing boron of density about {fraction (1/1000)} (˜1017 cm−3) into an amorphous silicon film containing oxygen of a certain density.
However, even if the aforementioned amount of boron is introduced to compensate an adverse effect of oxygen, it minimally affects solar cell characteristics. Improved interface characteristics should affect open circuit voltage mainly, but there is no change in open circuit voltage whether or not there is the compensation.